算法二刷day3

203.移除链表元素

class Solution {
public:
    ListNode* removeElements(ListNode* head, int val) {
        ListNode *dummyHead = new ListNode(0);
        dummyHead->next = head;
        ListNode *cur = dummyHead;
        while (cur->next != nullptr) {
            if (cur->next->val == val) {
                ListNode *tmp = cur->next;
                cur->next = cur->next->next;
                delete tmp;
            }else {
                cur = cur->next;
            }
        }
        return dummyHead->next;
    }
};

707.设计链表

class MyLinkedList {
public:
    struct LinkedNode {
        int val;
        LinkedNode* next;
        LinkedNode(int val) : val(val), next(nullptr){}
    };
 
    MyLinkedList() {
        _dummyHead = new LinkedNode(0);
        _size = 0;
    }
 
    int get(int index) {
        if ((_size - 1) < index || index < 0) return -1; //先排除下标无效情况
        LinkedNode *cur = _dummyHead->next;
        while (index--) {
            cur = cur->next;
        }
        return cur->val;
    }
 
    void addAtHead(int val) {
        LinkedNode *newNode = new LinkedNode(val);
        newNode->next = _dummyHead->next;
        _dummyHead->next = newNode;
        ++_size;
    } // 这里不要忘记size++
 
    void addAtTail(int val) {
        LinkedNode *cur = _dummyHead;
        while (cur->next != nullptr) {
            cur = cur->next;
        }
        LinkedNode *newNode = new LinkedNode(val);
        cur->next = newNode;
        ++_size;
    }
 
    void addAtIndex(int index, int val) {
        if (index > _size) return;
        if(index < 0) index = 0;
        LinkedNode *cur = _dummyHead;
        while (index--) {
            cur = cur->next;
        }
        LinkedNode *newNode = new LinkedNode(val);
        newNode->next = cur->next;
        cur->next = newNode;
        _size++;
    }
 
    void deleteAtIndex(int index) {
        if (index >= _size || index < 0) {
            return;
        }
        LinkedNode *cur = _dummyHead;
        while (index--) {
            cur = cur->next;
        }
        LinkedNode* tmp = cur->next;
        cur->next = cur->next->next;
        delete tmp;
        tmp=nullptr;
        _size--;
    }
 
    void printLinkedList() {
        LinkedNode *cur = _dummyHead;
        while (cur->next != nullptr) {
            cout << cur->next->val << " ";
            cur = cur->next;
        }
        cout << endl;
    }
 
private:
    int _size;
    LinkedNode *_dummyHead;
 
};

206.反转链表

class Solution {
public:
    ListNode* reverse(ListNode* pre, ListNode* cur) {
        if (cur == nullptr) return pre;
        ListNode* tmp = cur->next;
        cur->next = pre;
        return reverse(cur, tmp);
    }
    ListNode* reverseList(ListNode* head) {
        return reverse(nullptr, head);
    }
};